The positions of the satellites are determined on the basis of a network of ground tracking stations independent of the positioning receivers. These positions are communicated to the positioning receivers via the satellites themselves, by data transmission. The pseudo-distances are deduced by the positioning receivers from the apparent delays exhibited by the received signals relative to the clocks of the satellites, which are all synchronous.
Although the precision in knowing the positions of the satellites of the positioning system is independent of the performance of a positioning receiver, this is not the case for the precision of the pseudo-distance measurements, which depends on the precision of the measurement of the signal propagation times at the receiver.
Radio signals transmitted by satellites travel large distances and, since they are transmitted at limited power levels, reach the receivers with very low power levels that are buried in radio noise. To make it easier to receive them, it has been attempted to make them the least sensitive possible to narrow-band interference, by increasing their bandwidths by means of the band spreading technique. The current systems, and those intended in the near future, for satellite positioning used, for the radio signals transmitted by their satellites, the technique of band spreading by modulation with the aid of pseudorandom binary sequences, a technique known as DSSS (Direct Sequence Spread Spectrum). This DSSS modulation consists, after having arranged the information to be transmitted in the form of a sequence of binary elements with a regular datarate, in multiplying each binary information element by a pseudorandom binary sequence of markedly faster datarate. The band spreading obtained is proportional to the ratio of the datarate of the sequence of binary data elements to the datarate of the pseudorandom binary spreading sequence.
The information to be transmitted from the satellites, once placed in the form of a frequency-spread sequence of binary data items by DSSS modulation, are transposed in the transmission frequency range by modulation with a transmission carrier. To make it easier to measure the signal propagation times at a positioning receiver and to avoid the presence of isolated lines in the spectra of the signals transmitted by the satellites, each pseudorandom binary sequence used for frequency spreading consists of binary elements of the same duration, taken to be equal to integer multiples of the periods of the transmission carriers, whereas the various datarates and frequencies used within the satellites are synchronized and derive from a very precise common clock.
Upon reception, the binary information contained in a radio signal from a satellite of a positioning system is extracted by two demodulations often carried out in an interlinked manner, a first demodulation using a carrier generated locally by an oscillator controlled by a PLL (Phase Locked Loop) for transposing the signal received in broadband and a second demodulation using pseudorandom binary sequences generated locally by a pseudorandom binary sequence generator controlled by a DLL (Delay Locked Loop) for despreading the sequence of binary information items present in the received signal.
The propagation times of the received signals are manifested, at reception, by delays that affect the pseudorandom binary sequences present in the received signals and the carrier modulating the received signal.
The delays affecting the pseudorandom binary sequences are accessible, modulo the duration of one of their binary digits, from the control signals of the DLLs. The delays observed by these loops allow unambiguous measurements, or those of low ambiguity, of the propagation times of the pseudorandom binary sequences since the number of complete pseudorandom sequences passing over the signal paths is relatively small. This is referred to as code measurements.
For example, in the case of the GPS satellite positioning system, the shortest pseudorandom binary sequence, that used for satellite signal spreading of the C/A (Coarse/Acquisition Code or Clear/Acquisition Code) type, is composed of 1023 binary digits with a rate of 1023 MHz and a duration of one millisecond. Its total duration corresponds to a path of 300 km for a radio wave and allows measurements of distance modulo 300 km. The 1 microsecond duration of each of its binary digits permits a precision of the order of 0.1 microseconds in the measurement of its delay at reception, corresponding to a 30 meter path in the case of a radio wave. The ambiguity in the pseudo-distance measurements obtained from the pseudorandom binary sequence of a C/A code, due to the fact that measurements modulo 300 km are to be made, is easy to resolve as soon as the receiver receives from more than four satellites, as it is then possible to take various points on the same position from different sets from four satellites and to retain only the common solution. In the absence of such a possibility, the ambiguity may also be resolved using very coarse prior knowledge of the position. Such a measurement ambiguity does not arise with P-type satellite signals of the GPS system, which use, for spreading them, a pseudorandom binary sequence of 266.41 days duration, but these signals are not freely available to users.
The apparent delays of the transmission carriers are accessible, modulo the periods of these carriers, by the phase shifts displayed by the PLLs that control the local carrier generators. This is referred to as phase measurements. These measurements are very precise, but highly ambiguous. In the case of the GPS system, the satellite transmission signals lie within the 1.3 GHz and 1.5 GHz frequency bands and allow pseudo-distance measurement modulo 0.2 m, therefore highly ambiguous measurements, since the distance to the satellites is of the order of 20 000 km, precise to about 0.02 m.
It is conceivable to use, concurrently, the phase measurements and the code measurements on one and the same received signal coming from a positioning satellite and to apply a Vernier technique in order to improve the precision of the unambiguous code measurement by means of the very precise but highly ambiguous phase measurement. However, the very large difference between the measurement scales, namely the wavelength of the pseudorandom binary sequence of the spreading code and the wavelength of the transmission carrier, does not allow the precision of a phase measurement to be retained while resolving the ambiguity sufficiently to have easily useable pseudo-distance measurements.
It is usual for a satellite of a positioning system to transmit over at least two frequency ranges so as to allow a receiver to estimate the effect of the ionosphere on the ionospheric propagation speed and to take account of this in its pseudo-distance measurements. Thus, a GPS satellite transmits over two frequency ranges centered on 1575.42 and 1227.60 GHz. Within the 1575.42 GHz range, it provides two navigation services consisting of the transmission of the same ephemeris data over two independent channels spread in one of the channels by a C/A code and in the other by a P(Y) code. In the 1227.60 GHz range, it provides a single service, which may consist of the transmission of data spread by a C/A code or a P(Y) code, or of a single P(Y) code.
Currently, the envisaged satellite positioning systems are designed to offer several services corresponding to different levels of performance and of integrity in the pinpointing matched to the more demanding or less demanding requirements of different categories of users. This is the case with the new Galileo satellite position system that offers several services, all dual frequency (E1 & E5, E2 & E6, etc.), so as to allow ionosphere correction then to improve robustness in the case of jamming of one of the bands, some services being free-access services and others controlled-access services.
In the case of dual-frequency services, the signals are tracked independently over each band by PLL and DLL tracking loops. The pseudo-distance measurements, which rely on the delays observed by the PLL tracking loops, are limited in terms of precision by the width of each available band. Because of the independence of the tracking loops, any combined use of measurements obtained from signals of different bandwidths will result in a precision limited by the lesser of the two tracking loops.
What is sought in real time is an absolute measure of the distance between the satellite and the receiver that has a precision given by the frequency difference between the bands (a wavelength fraction of the difference). To do this, it is sought to define a method of improving the measurement precision that is more robust than the known phase Vernier method, which is as independent as possible of ionosphere effects, and making it possible, despite everything, to benefit from the frequency spacing between two limited-band signals, for example the signals E1 and E2.